SpaceX is trying to shorten the distance between the landing site and the launch site for shorter launch cycles, so the use of the controlled landing, but it seems also possible to land horizontally without losing this advantage.

To prevent a crash when reaching the ground, the sea or the barge, why not just switch, 30s before the landing, from thrust-controlled trajectory to an unpowered horizontal landing with parachutes and airbags)

$\begingroup$Rockets are already designed for (primarly) axial stress, pushing from below and pushing from aerodynamic drag on the nose (and skin). Having to deal with off-axis loads means they'll need more strength, which takes more weight, costs more, etc. Ultimately landing vertically is clearly possible, and all it seems to require are some control surfaces (light), control software (basically free), and landing legs. Just because it's easier to conceptualize doesn't mean it's better.$\endgroup$
– Nick TJan 28 '15 at 23:53

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$\begingroup$@NickT And additional fuel. Don't forget fuel. Vertical landing maneuvers require fuel for the retro rockets. Fuel is itself both heavy and expensive, and the horizontal landing that uses rockets only to reorient (which the vertical ones are doing too), and then perhaps for minor corrections, if that, on a mostly unpowered approach, is likely to use much less than the retro rockets of the vertical landing.$\endgroup$
– Matthew NajmonJan 31 '15 at 15:40

6 Answers
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Underlying all of SpaceX's decisions is the desire to go to other planets, especially Mars. For exploring the solar system, Elon Musk feels that precision landings are extremely important. The precision landing requirement means that you want to start slowing the spacecraft before you reach the surface, which points towards wings or rockets. Unfortunately, not every moon/planet in the solar system has an atmosphere capable of supporting flight (or at least, flight of heavy aircraft at reasonable speeds), which makes rockets a more appealing option.

$\begingroup$It also bears mentioning that a rocket landed vertically (if sufficiently developed that it did not require a launch pad for launch) can simply be refueled and taken off again without the need for anything else to happen; a horizontal landing would require equipment to put the rocket vertical again (unless it took off horizontally as well).$\endgroup$
– NeutronStarJan 27 '15 at 18:09

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$\begingroup$Along with this: Elon cranked up the difficulty level by making the first destination a barge, in the ocean, whose elevation is constantly changing. His rocket team is on expert mode.$\endgroup$
– NotMeJan 28 '15 at 17:53

It is unnecessary to use anything but the rockets. For the same reason a helicopter uses its engine and rotor to land. It would be a bit ridiculous to land a helicopter with parachutes and airbags, wouldn't it?

Boeing CST-100 will use parachutes and airbags to land, and Elon Musk made fun of it in an interview, saying that its drop test landing looked like a crash and that it is not the way to land a spaceship in the 21st century:

Vertical rocket powered landing is more reliable than parachutes and only adds fuel, no extra hardware mass. It also saves the runway and gives total flexibility to land anywhere anytime, also on airless bodies. It is obviously the best way to do it. The Russian Baikal concept takes the horizontal landing concept to its extreme, I think. Adding jet engines, wings, landing gear and whatnot to the rocket boosters. It's like putting a booster on an ordinary airplane.

To shorten the launch cycle, the landed first stage maybe could be lifted from the barge to land by a helicopter? The Mi-26 is capable of carrying 20 tons 800 kilometers; the dry weight of Falcon 9's first stage is 16 tons.

$\begingroup$"Vertical rocket powered landing is more reliable than parachutes". Interesting. For completeness, do you have figures?$\endgroup$
– minsJan 27 '15 at 10:06

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$\begingroup$@mins There aren't many rocket powered vertical landings to compare with, but a parachute is obviously more sensitive to weather and does not have the flexibility during the landing process that rocket engines have. Helicopter landings aren't very risky, but parachute jumps are. Failed airbags might be a reason for the failed Beagle lander on Mars, they never tested successfully on Earth before launch.$\endgroup$
– LocalFluffJan 27 '15 at 10:09

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$\begingroup$Great answer. The one thing I don't see addressed here is that parachutes rely on an atmosphere. They're pretty great for Earth but who knows where we'll be going.$\endgroup$
– MiniRagnarokJan 27 '15 at 14:24

$\begingroup$Doesn't the Dragon 2 use parachutes as a backup (iirc, the engines test-fire to verify they're working on the way down) and/or to bleed off some speed in the air?$\endgroup$
– Nick TJan 28 '15 at 23:57

SpaceX have tried using parachutes for landing the first stage, but they weren't happy with the results.
A landing with airbags is more complicated: you need to attach parachutes to at least two locations (top and bottom of the stage). You still don't have fine control over the landing location. You need to land on a barge to prevent soaking the stage in seawater (which would add a lot of processing time to clean the stage). A barge landing requires exact control.
Landing horizontally also means you need more space for the landing: in a vertical landing the footprint is 2x the length of the landing legs, in a horizontal landing you need an area at least as long as the stage.
Airbags have never been used for landings of this size: the largest airbags I know of were for the Mars rovers Spirit and Opportunity. For Curiosity, NASA switched to a powered landing because they didn't think they could make airbags work. The Falcon 9 first stage is at least 10x heavier than Curiosity.

$\begingroup$Add to that Mars gravity is much less than Earth's, if airbags won't work there they won't work here either.$\endgroup$
– GdDJan 27 '15 at 9:20

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$\begingroup$It's also worth mentioning that Spirit and Opportunity still used retrorockets before detaching from the parachute/aeroshell and landing on the airbags.$\endgroup$
– Pedro WerneckJan 28 '15 at 16:42

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$\begingroup$Anecdotally, I met a SpaceX guy hiking the other day, and he emphasized the same issue you pointed out concerning not wanting the whole thing immersed in sea water.$\endgroup$
– Ben CrowellJan 29 '15 at 23:26

Long ago the head of the now long defunct ROTRON project commented that "Wings are the most expensive form of recovery" - and then explained that by using a "rocket powered helicopter" for the in-atmosphere portion of ascent they were effectively delivering a set of wings to orbit at zero net energy cost so they could use them for descent. If your wings 'go along for the ride' without 'paying their way' as ROTRON's did, they represent a net expense on the mass budget. 1st stage recovery does not require taking your descent mechanism to orbit but is still a cost to orbitable payload. Given that orbited payload as a % of launch mass is typically ijn the 1%-5% range, every saved kg helps. (Every saved pound helps too :-) ).

The cost of using fuel in place of wings, parachutes, airbags etc is slightly more tankage, the fuel mass required and possibly added motor reliability and run-life.
Those who have pursued this path have always claimed that the gains outweigh the costs.
But, then, they would :-).

$\begingroup$The alternative landing method that I had I mind (before readings answers) was to switch from engine-controlled attitude to parachutes and airbags deployment at the very end, let's say 30s before landing.$\endgroup$
– minsJan 29 '15 at 6:02

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$\begingroup$@mins Flight mode transitions are difficult. In different regimes have different control systems. In effect you are putting a very complex and highly random operation 30s before you are about to crash. Its like pulling off a powerslide in a car at 500mph to try to park by a wall.$\endgroup$
– AronJan 29 '15 at 7:52

Rockets landing back vertically by series of retro burns are critical point in SpaceX reusability program. Even more, their human spaceflight program is based on concept pinpoint powered decent. A number of other companies work on propulsive atmospheric return. All they use for terminal descent the same controls used in ascent –vectored trust, usually by a gimbaled engine. This arrangement was very popular in the old Sci-Fi movies – the landing is simply ascent in reverse motion.

The practical reason to follow such intuitive idea is the attractive opportunity to recover the existing rockets with minimum modifications.

SpaceX leadership was confident that reliable recovering the rocket boosters is a step away after demonstration of controllable hovering and descent. In practice, it has proved more difficult. The test flights of the reusable booster in more realistic environment - reentering from suborbital trajectory disclosed a number of problems. Significant modifications of initial concept have been made but the success remains elusive.

from Defa 1960 Sci-Fi move

Contemporary launch vehicles are strictly optimized for axial stress at vertical launch. Although any attempt to use a system developed and refined for specific purpose (vertical ascent) for completely different one (vertical descent) will certainly confront once more the basic problems already solved during primary use development. To enable for pinpoint vertical landing the rockets designed to fly against target or to launch a spacecrafts will need to rethink and possibly re-solve the basic problems of propulsive flight to meet the new emerged requirements.

The argument between horizontal and vertical landing for reusable vehicles was conducted between noted space historian Henry Spencer and Mitchell Burnside Clapp who worked on both the DC-X and X-33 programs: